1. Field of the Invention
The invention relates to low pressure, low density alkylene interpolymers in carbon black concentrates to be used in blends of thermoplastic ethylene polymers from which medium density grades of pipe may be formed.
2. Description of the Prior Art
Plastic pipe made from thermoplastic ethylene polymers is usually characterized, in terms of the nominal density of the resin from which the pipe is formed, as being a low, medium or high density grade. The low density material has a density of about 0.910 to 0.925 grams per cubic centimeter, the medium density material has a density of about 0.926 to 0.940 grams per cubic centimeter, and the high density material has a density equal to or greater than 0.941 to about 0.959 grams per cubic centimeter.
The manufacture of medium density pipe making compounds has usually been accomplished by blending low density resin with high density resin so as to be able to provide a wider range of physical properties with the resulting blend of resins than historically could be easily provided by the use of individual medium density resins.
Natural ethylene polymers have a detrimental property in that they slowly degrade in air (oxygen) which degradation is accelerated by the presence of heat and/or ultraviolet radiation. Photo- and thermal-oxidation stabilization can be achieved by the addition of carbon black. In addition to their function as a light screen, carbon black particles may act as radical traps.
The use of carbon black, however, poses problems of dispersion, for in order that it be effective in inhibiting degradation of physical and chemical properties of ethylene polymers due to weathering, it must be uniformly distributed throughout the polymeric matrix. The effectiveness of carbon black as an inhibitor is directly proportional to the number of the dispersed carbon black particles and the uniformity of their distribution and inversely proportional to the size of the individual particles of carbon black in the matrix. But the finer the particles of carbon black, the greater the tendency to form mechanical aggregates of particles or "agglomerates". These agglomerates cause the final product to exhibit a reduced resistance to degradation, uneven pigmentation, and poor surface characteristics, e.g. roughness and lack of gloss. In addition, the presence of agglomerates on the surface creates processing difficulties. The net result of these agglomerates is an unsatisfactory product.
Heretofore, methods of introducing carbon black into ethylene polymers have been unable to produce pipe that meets the increasingly critical desires of the piping industry for improved gloss. A commonly used method involves sprinkling the desired amount of carbon black, as a powder, on the ethylene polymer composition while it is fluxed under conditions of high sheer on a differential two-roll mill until a mixture of the carbon black in the polymer is obtained. This method is slow and, therefore costly and cannot consistently produce large quantities of uniformly pigmented ethylene polymer which is substantially free of agglomerates. In addition, the problem of dusting inherent in this method is not only injurious to health but creates both fire and explosion hazards.
The above-described technique is adaptable to production of a highly concentrated mixture or masterbatch of carbon black and ethylene polymer. Unpigmented or natural ethylene polymer is added to this masterbatch in a later step to reduce the concentration of carbon black to that desired for the final product. Products made in this way show a definite increase in quality over those produced without the concentrate addition step.
During the production of extruded pipe, resin exits a die in tubular form and is drawn through the interior of a brass sizing sleeve which polishes the exterior surface of the extruded polymer pipe. As a result of the carbon black concentrate addition to the pipe blends, the interior, freely extruded surface of the ethylene polymer pipe always realized gloss characteristics significantly below and roughness characteristics significantly higher than that achieved on the exterior, polished surface of the extruded pipe.
Despite the versatility of being able to easily provide a wide range of extrudable medium density ethylene pipe making compounds by blending low density resin with high density resin, it has not been readily possible prior to the present invention, to provide an extrudable medium density pipe making composition from ethylene polymers to which a carbon black concentrate has been added which is entirely suitable for making extruded medium density pipe therefrom under present day high speed (10 to 100 feet per minute, 100-1,000 lbs. per hour at temperatures of greater or equal to 200.degree. C. and at pressures of greater or equal to 500 psi) pipe extrusion conditions and wherein the resulting pipe would have a high gloss, satin-like finish on the inside or freely extruded surface of the pipe concommitant with a notched low temperature embrittlement point of less than -65.degree. C. in the composition. Of course, smooth and glossy surfaces are important not only from an esthetic viewpoint but also for fluid flow considerations.
Commercially acceptable grades of carbon black filled medium density potable water pipe must meet the following criteria: ASTM pipe specifications in accordance with the PE-2306 classification.